It is a requirement of a wireless communication system to transmit a large amount of high quality multimedia data using a limited frequency. As a method for transmitting a large amount of data using a limited frequency, a multiple input and multiple output (MIMO) system was introduced. The MIMO system forms a plural of independent fading channels using a multiple antenna at receiving and transmitting ends and transmits different signals through each of transmitting antennas, thereby significantly increasing a data transmission rate. Accordingly, the MIMO system can transmit a large amount of data without expansion of a frequency.
However, the MIMO system has a shortcoming that the MIMO system is too fragile for inter-symbol interference (ISI) and frequency selective fading. In order to overcome the shortcoming, an orthogonal frequency division multiplexing (OFDM) scheme was used. The OFDM scheme is the most proper modulation scheme for transmitting data at a high speed. The OFDM scheme transmits one data row through a subcarrier having a low data transmission rate.
A channel environment for wireless communication has multiple paths due to obstacles such as a building. In a wireless channel environment having multi-paths, delay spray is generated due to the multiple paths. If a time of delay spray is longer than a time of transmitting a next symbol, inter-symbol interference (ISI) is generated. In this case, fading is selectively generated in a frequency domain (frequency selective fading). In case of using single carrier, an equalizer is used to remove the ISI. However, complexity of the equalizer increases if a data transmission rate increases.
The shortcomings of the MIMO system can be attenuated using an orthogonal frequency division multiplexing (OFDM) technology. In order to overcome the shortcomings of the MIMO system with the advantages of the MIMO system maintained, an OFDM technology was applied to a MIMO system having N transmitting antennas and N receiving antennas. That is, a MIMO-OFDM system was introduced.
FIGS. 1 and 2 are a block diagram schematically illustrating a multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) system. FIG. 1 is a block diagram of a transmitting end in the MIMO-OFDM system, and FIG. 2 is a block diagram of a receiving end in the MIMO-OFDM system.
Referring to FIG. 1, the transmitting side includes a serial/parallel (S/P) converter, a plurality of encoders 102, a plurality of quadrature amplitude modulation (QAM) mappers 103, a plurality of inverse fast fourier transform (IFFT) units 104, a plurality of cyclic prefix (CP) inserters 105, and digital/analog and radio frequency (D/A & RF) converting units 106. The S/P converter divides transmission data into a plurality of data rows before encoding the transmission data. The plurality of encoders 102 encode the data rows. After encoding, the plurality of QAM mappers 103 modulate the encoded data rows based on a predetermined modulation scheme such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16 QAM, and 64 QAM. The plurality of IFFT units 104 transform the modulated symbols into time domain signals. The plurality of CP inserters 105 insert a CP code for a guard interval into the time domain signals. Then, the plurality of D/A & RF converting units 106 convert the CP inserted digital signals to analog signals and covert the analog signals to RF signals. The RF signals are transmitted through an antenna.
Referring to FIG. 2, the receiving side includes a plurality of analog/digital and radio frequency (A/D & RF) converting units 107, a plurality of CP removers 108, a plurality of fast fourier transform (FFT) units 109, a MIMO receiver, a plurality of decoders 111, and a P/S converter 112. The plurality of A/D & RF converting units 107 convert RF signals to analog signals and convert the analog signals to digital signals. The plurality of CP removers 108 remove CP codes which were inserted for a guard interval and transfer the CP code removed signals to the FFT units 109. The plurality of FFT units 109 perform FFT on the input parallel signals which are the CP removed signals. The MIMO receiver 110 estimate transmission data symbols which are generated by FFT. The MIMO receiver 110 calculates a log likelihood ratio (LLR) from the estimated symbols. The plurality of decoders 111 decode each of data rows transferred from the MIMO receiver 110 and estimate the transmission data. The plurality of P/S converters 112 convert parallel data modulated by each decoder 111 to serial data.
The MIMO receiver 110 generally uses a decision feedback equalizer (DFE), zero forcing (ZF), minimum mean square error estimation (MMSE), and bell labs layered space-time (BLAST). As described above, the MIMO receiver has a problem of low performance although the MIMO receiver has a comparative simple structure compared to maximum likelihood detection (MLD).